There are many instances where it would be useful to be able to determine the location of an individual. For example, in an emergency situation, first responders (e.g., police officers, firefighters, miners) may need to determine where a person in distress is located. Military operations are another example where the ability to locate a soldier in distress is desirable.
The desirability of locating people in an emergency situation is exemplified by the implementation of the enhanced 911 ability in cellular telephones that allow the location of a cell phone being used for a call to the emergency 911 number to be located. Some of these enhanced 911 solutions rely on the Global Positioning System (GPS) to determine the location of the cellular telephone.
Of course, a cellular telephone is not practical in all situations, and will only work in areas which have cellular coverage. While cellular coverage is quite good in populated areas, many sparsely populated areas have little or no cellular coverage. Cellular telephones are also somewhat impractical from a global perspective due to differing frequency allocations and waveforms in use around the world. Another problem is that cellular and other systems require an extensive infrastructure to be deployed.
Reliance on GPS can also present difficulties. While GPS provides worldwide coverage, the GPS signals are easily blocked by heavy vegetation and structures, making GPS receivers unreliable inside buildings, underground, and in heavily vegetated areas. Moreover, GPS receivers tend to be relatively power hungry.
Accordingly, there remains a need for emergency locating systems than use a small, lightweight unit and that do not require an extensive infrastructure or GPS.